Battery drain on 6.9 diesel

We have a 1984 f350 6.9 diesel. If she sits for a day she won't start. We go through about 6 batteries a year. They lose the ability to start the truck. We have changed the alternator and we disconnect the positive terminals on both batteries any time it will sit more than a couple hours but it still loses battery charge (11.7 and 11.6 this morning), enough that we have to jump it. The current batteries are about two months old and when new have no problems starting the truck.

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